Either MacDonald's Government doesn't collapse before Representation of the People (No. 2) Bill makes it through, thus giving Britain AV in boroughs with over 200,000 people (and paving the way for STV), or James Henderson-Steward is able to convince the Liberal Party to fold into the National Liberal Party before the Woolton-Teviot Pact in early 1947. There were a lot of issues with JHS' proposal, such as the Libs hatred for the Nats, but a more unified effort to get the Liberals in could help greatly, such as Macmillan openly offering them PR for Big Cities in exchange for a unified 'anti-socialist front'.
This kills the birds of a unified Liberal/Conservative Party, as well as the electoral system with one stone. However Federalism is a bit of a sticky wicket given the Conservatives historic opposition to it. An earlier devolution through Labour (and what aspects of the Liberals opposing merged with the Tories folding in with the Labs) is most likely, I think. Maybe the Scottish Covenant Association becomes a louder voice in Parliament, possibly leading to the Government taking an all-or-nothing situation on Devolution?
Alternatively, going earlier, the Government of Scotland Bill 1913 passes, though this would require no/delayed First World War.